The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential.
In a four-wheel drive motor vehicle, a torque distribution ratio of front wheels and rear wheels is determined to coincide with a dynamic weight ratio of the front and rear wheels rather than a static weight ratio thereof, so that the power of the engine may be most effectively used at acceleration.
In a four-wheel drive motor vehicle based on a front-wheel drive vehicle with a front-mounted engine, which has the static weight ratio of 60(front):40(rear), the ratio of front torque T.sub.F to front wheels and rear torque T.sub.R to rear wheels is determined in the ratio of 50:50 which is the same as the dynamic weight ratio thereof. In a four-wheel drive motor vehicle based on a rear-wheel drive vehicle with a front-mounted engine, which has the static weight ratio of 50:50, the ratio of the front torque T.sub.F and the rear torque T.sub.R is determined in the same ratio of 40:60 as the dynamic weight ratio. Accordingly, a central differential comprising bevel gears is employed in the former vehicle in which the output torque of the engine is equally distributed, and a central differential having a simple planetary gear device is employed in the latter vehicle.
The former vehicle ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driving force of the vehicle is improved. However, when the vehicle makes a turn at high speed under the differential lock condition, all of the four wheels may slip (i.e. the vehicle spins) at the same time, causing difficulty in driving.
In order to ensure a driving stability of the vehicle, the torque to the rear wheels is set to a value larger than that to the front wheels by arranging the central differential comprising the simple planetary gear device, so that rear wheels may slip first. Thus, the vehicle can be safely driven because a driver immediately notices the slipping of the rear wheels before all of the four wheels slip at the same time.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. The output of a transmission is transmitted to a carrier of the planetary gear device. The torque is distributed to the front wheels through either a sun gear or a ring gear and to the rear wheels through the other gear. The torque to the front and rear wheels is unequally distributed at the ratio determined by the ratio of the pitch circles of the sun gear and the ring gear. The torque distribution ratio can not be changed with the large degree of freedom. Furthermore, it is difficult to change the diameters of the sun gear and the ring gear because of limitation of space in the transmission.
Further, in a power transmitting system having a transaxle type of a manual transmission, the central differential must be disposed in a space adjacent to a transfer shaft at the rear of the transmission so as not to reduce a minimum ground clearance and the space in the vehicle. Therefore, it is improper to use the central differential with the simple planetary gear device, because the diameters of the sun gears and ring gears and the torque distribution ratio can not be increased. Moreover, if a fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation, the distribution of torque to the rear wheels can not be set to a large value, so that the control range of the multiple-disk friction clutch becomes small. Consequently, a motor vehicle having good driveability and controllability of handling can not be provided.